def. Ice:
Water (H₂O) in a solid state. When naturally occurring, ice is considered a mineral. There are many forms of ice: lake ice, river ice, sea ice, snow, glaciers, ice caps, ice sheets, and frozen ground (such as permafrost).

If you ask a geologist what he or she considers to be Earth’s most important mineral, you will probably hear many different answers, depending on the person. Some might choose a common mineral, perhaps olivine or quartz or feldspar. Others might choose a rare, economically valuable mineral, perhaps gold or tantalite or monazite. Others might choose a mineral that is important for understanding the geological history of the Earth, perhaps zircon or spinel or serpentine or maybe even jimthompsonite. However, if you ask me what I consider to be Earth’s most important mineral, I think that I would reply that it is ice.

After all, how many minerals have an entire field of geology devoted to them? Glaciology is a very important field of geology that is devoted to the study of glaciers and, more generally, ice in all its forms.

And how many minerals cover vast expanses of the Earth? Sure, much of the mantle consists of olivine and much of the core consists of metal alloys, but no other mineral can compete with ice when it comes to Earth’s surface. Ice covers much of the Earth’s land and ocean surface, and the portions of the Earth that are covered with ice comprise the cryosphere. Some scientists even believe that during particularly cold time periods in the past, the entire Earth, a so-called “Snowball Earth”, was covered with ice, an all-encompassing cryosphere.

And how many minerals play such an important role in Earth’s climate? The formation of ice is a key aspect of Earth’s climate system. Very simply, there is more ice when Earth is colder and there is less ice when Earth is warmer. However, the transition from water to ice and ice to water can be complex. For example, the light color of ice creates an ice-albedo feedback loop. Climatologists are still working to unravel the details of the roles that ice plays in global climate.

And how many minerals are less dense than their liquid counterparts? The lower density of ice relative to water enables it to float on top of water, and this physical arrangement has a profound impact on the nature of the hydrosphere and biosphere. If ice did not float on top of water, then many natural bodies of water might freeze completely, never fully thawing in the heat of summer. The lower density and greater volume of ice (compared to water) also plays a role in geomorphology. The expansion of water when it freezes into ice contributes to the physical weathering of rocks. The freezing and thawing of water breaks up rocks and is also responsible for other geomoprhological processes, such as solifluction.

I’m sure that there are many other reasons why ice is an important mineral. However, I think that the ones I’ve listed above are sufficient to convince me. If anyone asks me what I consider to be Earth’s most important mineral, I will answer ice… although since I currently work as a diamond geologist I might broaden the definition to include both frozen water and the sparkly gemstones I study at work.

But what would you answer? What is Earth’s most important mineral? What about Earth’s second most important mineral?

I have to admit, until I gave some thought to this week’s geology word I don’t think that I would have answered ice. I probably would have answered olivine, since I like mantle geology. However, that’s because I don’t generally think of ice when I think of minerals. I guess that ice is so common, and also so unusual (compared to other minerals), that it doesn’t fit into my standard “this is a mineral” box. There’s no doubt about it, though: ice is a mineral, and it is an important one.

Of course, not all ice is a mineral. The ice that you find in ice cubes in your drink is technically not a mineral since minerals must be naturally occurring by definition. My husband, who grew up in South Africa and only saw snow for the first time in his 20s, has a nice way of categorizing ice: he calls the ice that you find in your refrigerator “domesticated ice” and ice that you find out in nature “wild ice”. Growing up in a landscape without ice, my husband finds the “wild ice” of New Hampshire, where I come from, fascinating. And I suppose he should. For while a frozen puddle on the street, a line of icicles on a rooftop, and a dusting of snow on the driveway may seem quite ordinary to someone who grew up in a cold climate, they are small pieces of a quite extraordinary cryosphere that is an integral part of the planet we call home.